In an I. S. Machine, a gob of molten glass is delivered to a blank mold in which a “parison” is formed. The formed parison includes the “finish”, i.e., the threads, etc. of the bottle to be formed which are at the bottom of the vertically standing parison. The parison also has an elongated vertical hole which extends upwardly from the finish. The finish is gripped by neck ring molds carried by a pair of closed neck rings. When the blank mold is opened, the neck ring is rotated 180 degrees to deliver the parison to the blow station where opposed blow molds will be displaced into a clamped closed position about the parison. The finish, and hence the parison, is supported by the closed blow molds. Reheat then takes place, i.e., the skin of the parison which had been cooled to preserve the shape of the parison, is heated by the inner heat of the parison. When this happens the parison will sag or stretch downwardly by virtue of the parison being suspended from the molds. When the parison stretches to the desired length, either compressed air is introduced into the mold via a blow head sitting on top of the blow molds and/or vacuum is applied through the walls of the blow mold to displace the parison into the form of the mold.
Often cooling air is directed upwardly through the side walls of the blow molds to remove heat from the molds. It has been suggested that such air tends to leak into the blow mold and heated by the parison creates a large pressure that tends to collapse the parison so that instead of a continuous hole the hole is closed at some central location bringing one side of the parison into engagement with the other side of the parison. Should this happen, a “birdswing” can form when the parison is blown which is a defect requiring the rejection of the ultimately formed bottle.
To avoid birdswings, operators conventionally delay the start of cooling air until the parison has been blown but this slows down the cycle time and this is undesirable.